The application of a sinusoidal voltage (AC) to an electromagnet has significant advantages over applying a current directly (DC). However, the leakage inductance and coil resistance of the magnet will result in a force-gap characteristic that exhibits a negative spring rate.
A negative spring rate for a force generator makes it more difficult to use in a control system than one with a flat (zero) spring rate or a positive spring rate. For example, an electromagnet without stabilization cannot be used to levitate a mass. A magnet with positive spring rate can successfully levitate an iron mass.